Acoustic surface wave transmission device with amplification

ABSTRACT

A signal transmission device includes a piezoelectric substrate upon which surface waves are launched by an input transducer which may, for example, be composed of a pair of interleaved combs of conductive electrodes. An output transducer responds to the surface waves to develop an output signal. The output transducer includes a plurality of strips of semi-conductive material individually disposed across the path of the surface waves, with successive strips spaced apart by one acoustic wavelength. Spaced from each strip by an effective 1/2 wavelength distance are at least one and preferably a plurality of spaced conductive ribbons disposed end-to-end across the path of the surface waves. A plurality of conductive bridges individually join different portions of each strip to different portions of each associated ribbon. Across the opposing ends of the strips are connected a source of current and a load which derives signals from the currents in the strips that are modulated by the surface waves.

United States Patent Green 51 July 11,1972

[54] ACOUSTIC SURFACE WAVE TRANSMISSION DEVICE WITH AMPLIFICATION Mino Green, London, England Zenith Radio Corporation, Chicago, Ill.

June 21, 1971 [72] inventor:

[73] Assignee:

[22] Filed:

211 Appl. No.: 155,006

Primary Examiner-Roy Lake Assistant Examiner-Darwin R. I-lostetter Attorney-John H. Coult and John J Pederson [57] ABSTRACT A signal transmission device includes a piezoelectric substrate upon which surface waves are launched by an input transducer which may, for example, be composed of a pair of interleaved combs of conductive electrodes. An output transducer responds to the surface waves to develop an output signal. The output transducer includes a plurality of strips of semi-conductive material individually disposed across the path of the surface waves, with successive strips spaced apart by one acoustic wavelength. Spaced from each strip by an effective wavelength distance are at least one and preferably a plurality of spaced conductive ribbons disposed end-to-end across the path of the surface waves. A plurality of conductive bridges individuaily join different portions of each strip to different portions of each associated ribbon. Across the opposing ends of the strips are connected a source of current and a load which derives signals from the currents in the strips that are modulated by the surface waves.

5 Claims, 4 Drawing figures Load Patented July 11, 1972 FIG. 1

4 jnvenror Mlno Green Aflom High Voltage Source ACOUSTIC SURFACE WAVE TRANSMISSION DEVICE WITH AMPLIFICA'I'ION BACKGROUND OF THE INVENTION The present invention pertains to signal transmission devices. More particularly, it relates to surface wave amplifiers in which amplification is obtained by field-efiect action occurring in an output transducer.

It has long been known that signal energy may be transmitted by acoustic waves. Particular attention has been given in recent years to devices in which the signal energy is transmitted in the form of surface waves. The basic nature, and the principles involved in in the operation, of such surface wave devices are described in detail in co-pending application Ser. No. 721,038, filed Apr. 12, 1968, now U.S. Pat. No. 3,582,838 by Adrian DeVries and assigned to the same assignee as the present application.

Because of losses which inherently occur in the use of a surface wave transmission device, various attempts have been made to achieve amplification in the same device. One prior approach has comprised including a film of semi-conductive material adjacent to the wave-propagating surface and causing charge carriers to travel in the semi-conductor film at a velocity slightly greater than the velocity of the acoustic waves in their medium. Energy may be given up from the charge carriers to the acoustic waves as the result of which it becomes possible to obtain a degree of amplification. However, difficulties in connection with appropriate correlation of the difierent material parameters involved have impeded the successful exploitation of this approach to surface-wave amplifiers.

It is, accordingly, a general object of the present invention to provide a new and improved surface wave signal transmission device in which amplification is obtained in a manner overcoming the aforenoted difficulties.

It is another object of the present invention to provide amplification in a surface-wave device in a manner which permits its fabrication by the use of state-of-the-art techniques of fabrication.

A further object of the present invention is to provide a structural modification of such a device which enhances its level of amplification and permits wider flexibility in the choice of suitable constituent materials.

The invention, therefore, is directed to a signal-transmission device in which acoustic surface waves propagate along a piezoelectric substrate. The device includes signal developing apparatus having at least one strip of semi-conductive material disposed on the substrate across the path of the acoustic waves. A conductive ribbon is spaced from the strip by a distance effectively equal to one-half the wavelength of the waves. Coupling a portion of the strip to a portion of the ribbon is a bridge also of conductive material. A source of current is connected across the opposing ends of the strip. Finally, a load is coupled across the opposing ends of the strip to derive signals from the current in the strip that is modulated by the acoustic waves.

The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIG. I is a diagrammatic view of a surface-wave filter which also includes an element basically capable of developing an amplified output signal in response to acoustic surface waves;

FIG. 2 is a cross-sectional view taken along the lines 2--2 in FIG. 1;

FIG. 3 is a diagrammatic view of a preferred form of acoustic signal transmission device; and

FIG. 4 is a cross-sectional view taken along the line 4-4 in FIG. 3 and including additional circuitry preferably included in the device in FIG. 3 but omitted from that figure for the purpose of clarity.

In FIG. 1, a signal source 10 is connected across an input transducer 12 mechanically coupled to one major surface of a body or substrate 13 of piezoelectric material which serves as an acoustic-surface-wave propagating medium. An output or second portion of the same surface of substrate 13 is, in turn, mechanically coupled to an output transducer 14 across which a load 15 is coupled. Ignoring for a moment the additional structure shown between transducers 12 and 14, FIG. 1 illustrates one form of surface wave integratable filter of a kind described in more detail in the aforesaid DeVries application.

Transducers l2 and 14 in this simplest arrangement are identical and are individually constructed of two comb-type electrode arrays. The conductive teeth of one comb are interleaved with the teeth of the other. The combs are of a material, such as gold or aluminum, which may be vacuum deposited on a smoothly-lapped and polished planar surface of the piezoelectric body. The piezoelectric material is one, such as PZT, quartz or lithium niobate, that propagates acoustic surface waves. The distance between the centers of two consecutive teeth in each transducer is one-half of the acoustic wavelength in the piezoelectric material of the signal wave for which it is desired to achieve maximum response.

Direct piezoelectric surface-wave transduction is accomplished by the spatially periodic interdigital electrodes or teeth of transducer 12. A periodic electric field is produced when a signal from source 10 is fed to the teeth and, through piezoelectric coupling, the electric signal is transduced to a traveling acoustic wave on substrate 13. This occurs when the stress components produced by the electric field in the substrate are substantially matched to the stress components associated with the surface-wave mode. Source 10, for example the radio-frequency portion of the tuner of a television receiver, produces a range of signal frequencies, but due to the selective nature of the arrangement only a particular frequency and its intelligence-carrying sidebands are converted to surface waves. Those surface waves are transmitted along the substrate to output transducer I4 where they are converted to an electric signal for application to load 15 which in a typical television receiver may be a subsequent stage of the aforementioned tuner.

In a television tuner embodiment utilizing a lithium niobate substrate, the teeth of both transducers 12 and 14 are each about four microns wide and are separated by a center-tocenter spacing of eight microns for the application of a radiofrequency signal at 211.25 MHz. The spacing between transducers l2 and 14 in that example is on the order of 60 mils and the width of the wavefront is approximately 0.1 inch. This structure of transducers l2 and 14 together with substrate 13 can be compared to a cascade of two tuned circuits with a resonant frequency of approximately 211 MHz. The resonant frequency or frequency of maximum response is determined, at least to a first order, by the spacing of the teeth.

The potential between any given pair of successive teeth in transducer 12 produces two waves traveling along the surface of substrate 13 in opposing directions perpendicular to the teeth. When the center-to-center distance between the teeth is one-half of the acoustic wavelength of the wave at the desired input signal frequency or is an odd multiple thereof, relative maxima of the output waves are produced by piezoelectric transduction in transducer 12. For increased selectivity, additional electrode teeth may be added to the comb patterns of transducers l2 and 14. Further modifications and adjustments are described and others are cross-referenced in the aforementioned co-pending application for the purpose of particularly shaping the response presented by the device to the transmitted signal. In any event, the overall operation is such that electrical signals are converted into surface waves which, after traveling across the surface of substrate 13, are transduced back into electrical signals at transducer 14 and applied to load 15.

A C-shaped electrode 17 is located on substrate 13 intermediate transducers l2 and 14 and disposed across the path of the acoustic surface waves. Terminals l8 and 19 are connected respectively to the bottom and top or opposing portions of the electrode structure. For purposes of explanation and definition, it will be observed that electrode 17 is composed of a strip 20 fractional portions of which (in this case its two end portions) are joined by bridges 21 to fractional portions of ribbons 22 that are spaced in end-to-end alignment across the acoustic wave path. Ribbons 22 are spaced longitudinally from strip 20 by a distance at least efl'ectively equal to one-half the wavelength of surface waves propagating in the surface of substrate 13.

It may first be assumed that all portions of electrode 17 are formed entirely of a semi-conductive material in which charge carriers (electrons or holes) are free to drift in the presence and under the influence of an applied field. FIG. 2 depicts, by means of plus and minus signs 24 within substrate 13, electric field peaks generated in the piezoelectric material under electrode 17 as a result of the propagation of surface waves; the alternate plus and minus signs correspond to the peaks and valleys of those waves. Remembering that strip 20 is apaced onehalf wavelength from ribbons 22 as shown in FIG. 2, it will be apparent that the alignment of negative and positive peaks of field intensity, respectively, beneath strip 20 and ribbons 22 results in charge-carrier movement between the strip and the ribbon by way of bridges 21. Consequently, the strip becomes polarized with respect to the ribbons in a sense reciprocal to that of the field developed within the substrate. That is, the fields induced into electrode 17 as a result of piezoelectric action induced by the surface waves cause a charge carrier imbalance to exist within electrode 17 and between strip 20 and ribbons 22. A half-cycle later, in terms of the movement of the acoustic waves, there is a reversal of the different plus and minus signs shown in FIG. 2 so that the charge-carrier balance within electrode 17 is reversed in position. With continued travel of the surface waves under electrode 17, the charge-carrier level in strip 20 continually oscillates between one polarity and the other. In the presence of a voltage source causing an electric current to be conducted between terminals 18 and 19 through strip 20, the amplitude of that current is modulated by the changing charge-carrier level in the strip. In consequence, the combination of strip 20 and ribbons 22 acts upon the flow of current between terminals 18 and 19 like a field-effect transistor, and the fields induced in strip 20 from the substrate as a result of the wave motion act in the same manner as the field produced by the gate electrode of such a transistor.

As specifically shown in FIG. 1, electrode 17 serves as an additional output transducer or pick-up electrode capable of converting a portion of the acoustic wave energy into an output signal while, at the same time, affording a degree of amplification of that signal. As a first modification of the structure of FIG. 1, it will have become apparent that output transducer 14 may be omitted and load 15 may be connected directly to terminals 18 and 19, provided that the load includes a source of current that will flow between terminals 18 and 19. As a practical matter, however, it can be shown that, at least with conventionally available materials and at frequency ranges usually employed in communications equipment, electrode 17 as so far described will not enable a sufliciently fast response time of movement of the charge carriers to permit the achievement of a significant degree of amplification. To the end of reducing that response time, it is preferred that bridges 21 and ribbons 22 be composed of a highly conductive material, in which case only strip 20 is composed of semi-conductive material. Consequently, when strip 20 has induced in it a modulating charge of a singular polarity, so that its conductivity is changed with resultant modulation of the applied current, ribbons 22 constitute reservoirs to which excess charge is readily transferred by way of bridges 21. By being located a half wavelength away from strip 21, the reservoirs (ribbons 22) are correctly polarized by the underlying acoustic wave action for receipt of the excess charge. The response is speeded up because of the direct exit provided by bridges 21 for the excess carriers as compared, for example, to

the longer time of response were those excess carriers to be removed by way of one or the other of external terminals 18 and 19. In effect, the C-shaped arrangement of electrode 17 is such that each end of strip 20, in cooperation with respective one of ribbons 22, constitutes a field-effect transistor with the two transistors being connected in series between terminals 18 and 19.

To the end of improving upon the basic mechanism described in connection with electrode 17 in FIG. 1, the embodiment of FIG. 3 is preferred. As shown therein, signal source 10 is connected to an input transducer 30 disposed on a piezoelectric substrate 31 and again composed of a pair of interleaved combs of conductive electrodes or teeth. In the same way as before, transducer 30 launches acoustic waves that propagate along substrate 31 to an output transducer 32 which serves to develop electrical signals that are fed to a load 33 across the opposite ends of 34 and 35 of which an output signal may be derived. Included within output transducer 32 are a plurality of semi-conductive strips 38 the opposite ends of all of which are electrically connected in combination, and in this case in parallel, by electrode elements 39 and 40 across which load 33 is connected. Spaced from each strip 38, by a distance effectively equal to one-half of the acoustic wavelength, are a plurality of ribbons 42 that in turn, are spaced end-to-end across the path of acoustic wave propagation. A corresponding plurality of conductive bridges 43 individually connect respective different fractional portions of the associated strips to respective different ones of the ribbons. Finally, a source of unidirectional current, represented by a battery 45, is connected across the opposing ends of strip 38, in this case by being connected in series between load 33 and electrode elements 40.

In operation, the device of FIG. 3 functions in the same manner as that of FIG. 1. The current caused to flow within each of semi-conductive strips 38 is modulated by the action of the acoustic waves propagating in the underlying piezoelectric substrate. Each difierent portion of strip 38 co-acts with its associated ribbon 42 as a field-efiect transistor so that, as illustrated, each strip serves as four field-effect transistors connected in series. The current source need not be unidirectional. Using a radio-frequency current source, the device may instead function as a modulator or mixer.

Generally speaking, an increase in amplification is obtained by choosing a semi-conductive material that exhibits a lower charge-carrier concentration. At the same time, that direction of choice leads to slower response times, and hence to lower upper-frequency limits. For the purpose of raising those frequency limits, and hence speeding the response, the number of individually different ribbons 42 placed end-to-end adjacent to each semi-conductive strip is maximized. While, in principle, only a single semi-conductive strip, with an associated ribbon and bridge, is all that is necessary, a plurality of strips 38 each with a plurality of ribbons 42 are employed in FIG. 3 for the purpose of enhancing the proportion of the oncoming acoustical energy that is converted into electrical output signals. Also, the parallel combination of a plurality of semi-conductive strips 38 serves correspondingly to reduce the net output impedance presented to load 33.

To further increase the power in the signal delivered to load 33, a unidirectional electric field preferably is caused to extend transversely through strips 38. The use of such a field pemiits a decrease in the density or concentration of charge carriers within the medium of the semi-conductive strip. Accordingly, as shown in FIG. 4, a conductive layer 47 is deposited on the underside of substrate 13 opposite strips 38. A source 48 of high-voltage is connected between layer 47 and electrode 40, as a result of which a unidirectional electric field temiinates on strips 38. This arrangement is shown oruy in FIG. 4 in order to preserve the clarity of FIG. 3.

The purpose of the electric field created between semi-conductor strips 38 and layer 47 may become more apparent by a detailed consideration of some of the relationships involved. It is instructive to compare the signal power that may be developed by transducer 32 of FIG. 3 with that obtainable from a passive output transducer, such as output transducer 14 in FIG. I. As between the active and passive output transducers, it can be shown that the power gain is expressed by the relationship:

P /P Va ep( nq0)v, l. where P is the power of the signal developed by transducer 32=of FIG. 3, P is the signal power developed by transducer 14 in FIG. I, e is the permittivity of the substrate, n is the charge carrier density or concentration within the semi-conductor material of strips 38, q is the charge on each carrier, 0 is the thickness of strips 38, p is the direct-current power density per unit area and v is the surface-wave velocity. The quantity (nq0) represents the total movable charge per unit area present in semi-conductor strips 38. Further, it can also be shown that the phase delay wt in each fractional portion of strips 38 may be expressed:

achieve a phase delay less than one radian, requiredfor highfrequency operation, it thus becomes necessary that:

(kdx) (2) (kdx) (2nq0; -)/(ev). 3. Noting that the quantity (nq0p.) represents the sheet conductance of the semi-conductive material, it becomes evident that the particular semi-conductor to be selected should be one that exhibits comparatively high mobility of its charge carriers in order to obtain sufificiently fast response. At the same time, the material also should exhibit a relatively low chargecarrier density in order to increase the power gain as expressed by equation (1). To these ends, preferred semi-conductor materials include those whose chemical formulation is of the form (Cd, Zn As or Cd,Pb Te. Even then, it is still preferred also to employ the additional transverse field of FIG. 4 in order further to lower the value it of charge-carrier concentration or density.

Involving essentially only the disposition of perhaps as few as two different materials in a particular pattern upon the surface of the piezoelectric substrate, it will be understood that devices of the kind described herein may be fabricated by the use of techniques now well known in connection with the manufacture of integrated circuitry. By achieving amplification in the process of transducing the acoustic wave energy into electrical output signals, loss otherwise occasioned by the employment of surface wave devices is compensated. The

further refinement of the additional electric field, extending transversely into the semi-conductive medium in the surfacewave amplifier, enables either an attendant increase in the degree of amplification or a greater flexibility in the choice of materials, or both.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made therein without departing from the invention in its broader aspects. The aim of the appended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In a signal transmission device in which acoustic surface waves propagate along a piezoelectric substrate, signal developing apparatus comprising:

a strip of semi-conductive material disposed on said substrate across the path of said acoustic waves;

a ribbon of conductive material also disposed on said substrate across the path of said acoustic waves and spaced from said strip by a distance effectively equal to one-half the wavelength of said waves;

a bridge of conductive material coupling said strip to said ribbon;

a source of current connected across the opposing ends of said strip;

and means for coupling a load across the opposing ends of said strip to derive signals from said current modulated in said strip by said acoustic waves.

2. As signal transmission device as defined in claim 1 which includes a plurality of said strips disposed on said substrate across the path of said acoustic waves with successive ones of said strips spaced from one another efi'ectively by one wavelength of said waves, in which each of said strips is correspondingly coupled to a different one of a plurality of said ribbons by a different one of a plurality of said bridges, and in which said source and said load are connected and coupled, respectively, across the opposing ends of all of said strips in combination.

3. A signal transmission device as defined in claim 1 which includes a plurality of said ribbons spaced end-to-end across said path, and a plurality of said bridges individually coupling difierent portions of said strip to different ones of said ribbons.

4. A signal transmission device as defined in claim 1 which further includes means for establishing a unidirectional electric field extending transversely into said strip of semiconductive material, the intensity of said field decreasing the density of charge carriers in said strip.

5. A signal transmission device as defined in claim 4 which includes, as one pole of said field, a conductive layer disposed on a surface of said substrate opposite said strip. 

1. In a signal transmission device in which acoustic surface waves propagate along a piezoelectric substrate, signal developing apparatus comprising: a strip of semi-conductive material disposed on said substrate across the path of said acoustic waves; a ribbon of conductive material also disposed on said substrate across the path of said acoustic waves and spaced from said strip by a distance effectively equal to one-half the wavelength of said waves; a bridge of conductive material coupling said strip to said ribbon; a source of current connected across the opposing ends of said strip; and means for coupling a load across the opposing ends of said strip to derive signals from said current modulated in said strip by said acoustic waves.
 2. As signal transmission device as defined in claim 1 which includes a plurality of said strips disposed on said substrate across the path of said acoustic waves with successive ones of said strips spaced from one another effectively by one wavelength of said waves, in which each of said strips is correspondingly coupled to a different one of a plurality of said ribbons by a different one of a plurality of said bridges, and in which said source and said load are connected and coupled, respectively, across the opposing ends of all of said strips in combination.
 3. A signal transmission device as defined in claim 1 which includes a plurality of said ribbons spaced end-to-end across said path, and a plurality of said bridges individually coupling different portions of said strip to different ones of said ribbons.
 4. A signal transmission device as defined in claim 1 which further includes means for establishing a unidirectional electric field extending transversely into said strip of semi-conductive material, the intensity of said field decreasing the density of charge carriers in said strip.
 5. A signal transmission device as defined in clAim 4 which includes, as one pole of said field, a conductive layer disposed on a surface of said substrate opposite said strip. 